In aircraft, there is a need to provide a combustibly inert material for filling void space in fuel tanks to maintain a required amount of pressure in the fuel tanks and at the same time minimize any chance of fire in the fuel tanks. There is also a need to provide an oxygen-rich gas for the crew to breathe and for aircraft services that require oxygen. An example of such a service is an integrated power unit. Conventional methods for providing the inert material include placing rigid foams in the fuel tanks and providing inert gas to the fuel tanks from a storage container or containers aboard the aircraft. In the latter case, the inert gas is generated on the ground and stored in such containers in gas or liquid form. The containers are then loaded onto the aircraft and the stored fluid is used as required. The containers must be replenished between flights. The conventional approach to providing a source of oxygen-rich gas is basically the same as the latter approach to providing inert material, with containers of oxygen-rich gas or liquid being loaded onto the aircraft and replenished between flights.
These conventional approaches have a number of serious disadvantages, including relatively high life cycle costs and relatively complicated maintenance requirements. In addition, the storage of expendable liquids or gases on an aircraft requires a good deal of space and adds to the weight of the aircraft, thus reducing the overall efficiency of the aircraft. The storage of high pressure gaseous oxygen or liquid oxygen also presents a high risk of combustion is a leak occurs and there is any combustible material nearby.
The need to avoid the disadvantages associated with conventional approaches has led to an interest in on-board generation of combustibly inert gases and on-board generation of oxygen-rich gas. To date, the generating systems that have been proposed and/or put into practice either generate only one of the required inert gas and oxygen-rich gas or generate such gases separately. Combustibly inert gas is produced by processing engine bleed air with an inert gas generator which is a gas separation module based on either a selectively permeable membrane or a molecular sieve. The system for on-board generation of oxygen-rich gas usually process engine bleed air with an on-board oxygen gas generator which is based on a molecular sieve.
The major problem with the permeable membrane and molecular sieve gas separation modules is that they do not operate very efficiently, especially when they are required to yield a high purity product gas. Efficiency is defined as the ratio of the product gas to the supply gas. Oxygen generating systems generally have an efficiency of 2% to 10%, and inert gas generating systems generally have an efficiency of 20% to 40%. These relatively low efficiencies require the generating means to be relatively large and heavy in order to deliver the necessary product flow rates. In addition, a relatively large amount of bleed air must be supplied to the units to obtain the necessary amount of product gas. A major portion of the supply bleed air ends up being discarded as waste gas. The problems of increased weight, increased space requirements, and increased bleed air demands are especially troublesome in modern fighter aircraft. In such aircraft, even small increases in weight are significant, space limitations are particularly severe, and engine performance is particularly sensitive to removal of bleed air to other portions of the aircraft.
The patent literature includes a fairly large number of systems for processing a supply gas having two or more components to obtain a product gas enriched in one of such components. Each of the following United States patents discloses a system in which a molecular sieve or sieves are used to obtain an oxygen-rich product gas and/or a nitrogen-rich product gas from air: U.S. Pat. No. 3,102,013, granted Aug. 27, 1963, to C. W. Skarstrom; U.S. Pat. No, 3,796,022, granted Mar. 12, 1974, to G. Simonet et al; U.S. Pat. No. 3,880,616, granted Apr. 29, 1975, to W. P. Myers et al; U.S. Pat. No. 3,957,463, granted May 18, 1976, to G. M. Drissel el al; U.S. Pat. No. 4,011,065, granted Mar. 8, 1977, to H. Munzner et al; U.S. Pat No. 4,026,680, granted May 31, 1977, to J. J. Collins; U.S. Pat. No. 4,272,265, granted June 9, 1981, to F. P. Snyder; U.S. Pat. No 4,322,228, granted Mar. 30, 1982, to W. P. Myers et al; U.S. Pat. No. 4,349,357, granted Sept. 14, 1982, to G. K. Russell; U.S. Pat. No. 4,386,945, granted June 7, 1983, to P. J. Gardner; and U.S. Pat. No. 4,431,432, granted Feb. 14, 1984, to T. Amitani el al.
In the system described in the Gardner patent, two parallel columns of gas absorbent beds are provided. Each column includes a nitrogen selective molecular sieve, an oxygen selective molecular sieve and a plenum. Air from a compressor is simultaneously fed into each of the sieves in one of the columns. A portion of the product flowing through the oxygen selective molecular sieve in the column being supplied is flowed through the oxygen selective molecular sieve in the other column to desorb oxygen. The gas containing the desorbed oxygen flows out of the sieve and is stored in the plenum. When the beds of the molecular sieves in the columns being supplied are saturated, the air from the compressor is supplied to the second column and the first column goes through a desorption cycle. The air fed to the nitrogen selective molecular sieve is fed through the plenum, which contains the oxygen enriched gas obtained from the oxygen selective molecular sieve during the desorption cycle. The feeding of the compressed air through the plenum enriches the feed air supply to the nitrogen selective molecular sieve.
U.S. Pats. No. 3,149,934, granted Sept. 22, 1964, to H. Z. Martin, and 3,150,942, granted Sept. 29, 1964, to S. Vasan each disclose a system in which molecular sieves are used to obtain a hydrogen-rich product gas. U.S. Pat. Nos. 3,406,014 granted Oct. 15, 1968 to S. A. Guerrieri, and 4,287,170, granted Sept. 1, 1981, to D. C. Erickson each disclose a chemical oxidation-reduction process for separating air into oxygen and nitrogen. U.S. Pat. No. 3,891,411, granted June 24, 1975, to G. M. Meyer discloses a system for extracting nitrogen from a mixture of nitrogen, carbon dioxide, and water on board a ship.
The systems described above and the systems disclosed in the above-cited patents, as well as the prior art cited in such patents, should be carefully considered for the purpose of putting the present invention into proper perspective relative to the prior art.